This invention relates generally to medical implant devices, and more particularly to a biocompatible implant that is deformable at elevated temperatures and relatively rigid at body temperature.
A natural joint in the human body such as a hip joint may undergo degenerative changes due to a variety of etiologies. When these degenerative changes become advanced and are irreversible, it may ultimately become necessary to replace the natural joint with a prosthetic joint. Such a prosthetic joint is often formed from a high strength material that is not only able to accommodate the various loading conditions that the artificial joint may encounter, but is also biocompatible with the human body. Examples of such high strength materials used for the manufacture of prosthetic joints include metal alloys such as titanium or cobalt chrome alloys, metal alloys having metallic porous coatings secured to the outside thereof, and composite materials.
The general procedure that is used to replace certain natural joints with prosthetic joints will now be described with specific reference to a hip joint replacement. When implantation of a hip joint prosthesis becomes necessary, the head of the natural femur is resected. The acetabulum is then reamed so as to receive the acetabular component of the hip joint prosthesis. After the acetabular component has been implanted in the acetabulum, the intramedullary canal of the femur that is used for receiving and supporting the stem portion of the prosthetic hip is then reamed and/or rasped to form a cavity. A suitable adhesive such as bone cement is then introduced into the cavity. The stem portion of the prosthetic hip is then inserted into the cavity formed in the intramedullary canal, so that the stem portion becomes secured to the bone surrounding the cavity by the bone cement. A proper bonding of the external surface of the stem portion of the prosthetic hip to the wall of the cavity formed in the intramedullary canal requires that the bone cement be pressurized. This pressurization allows the bone cement to interdigitate with the wall of the intramedullary canal, as well as the external surface geometry of the stem portion of the prosthetic hip.
Under certain circumstances, it is desirable to limit or restrict the flow of bone cement from the area of the cavity immediately adjacent to the stem portion of the prosthetic hip into the lower portion of the cavity that is below the stem portion. Various methods have been used to restrict the flow of bone cement into the lower portion of the cavity. For example, fragments of biocompatible material such as hardened cement or bone have been inserted into the cavity which would then restrict the flow of bone cement into the lower portion of the cavity. In addition, permanent polyethylene plugs placed within the cavity have also been used to restrict the flow of bone cement into the lower portion of the cavity.
While these methods for restricting the flow of bone cement into the lower portion of the cavity have been successful, there are nevertheless various aspects of these methods that could be improved. For example, it is often difficult to remove hardened cement or bone fragments from the cavity, which is often necessary when the joint prosthesis is to be replaced by another prosthesis. In addition, polyethylene plugs are relatively rigid and therefore often tend to be relatively difficult to deform to the desired shape for easy placement within the cavity of the intramedullary canal. While the structural geometry of polyethylene plugs can be changed to improve the ease with which the plug can be inserted, such a plug may have a tendency to slide within the cavity when the cavity is pressurized so as to decrease the ability of the plug to maintain pressure.
The use of bone plugs made from bioresorbable materials has also been described in literature and have the advantage of being absorbed by the body over a period of time so as to allow for bone or fibrous material to grow into the space previously occupied by a bioresorbable plug. Such bioresorbable plugs are described as being made from a polymer such as polylactic acid. While these bioresorbable plugs may be capable of limiting the flow of bone cement into the lower portion of the cavity, they are presumably relatively rigid which would limit their ability to be easily manipulated into the desired location within the cavity unless they are made in a variety of sizes.
A need therefore exists for a bioresorbable implant that restricts the flow of bone cement into the lower pollution of a cavity formed in the intramedullary canal when the bone cement is pressurized, yet is sufficiently deformable so as to allow the bioresorbable implant to be positioned and manipulated into the proper location within the cavity with relative ease without the need for a relatively large number of sizes.